Hi! Thanks for a good question!
Although the arachnoid cyst
is not directly responsible for narrowing of the aqueduct, it is responsible for the symptoms, which are attributed to raised ICP (Intracranial pressure) due to the SOL (Space occupying lesion).
So, as has been advised by your surgeon, the choice of endoscopic third ventriculostomy- ETV would be the most appropriate management, although the arachnoid cyst is smaller in size.
Thanks again & wishing you the best possible treatment & early recovery!